98 



individuals which present intermediate characters. The majority 

 of the individuals were taken in a living condition, though many 

 empty shells were found. The conditions of the examination of the 

 plankton and the opacity of many of the shells made it impossible 

 to distinguish the dead shells in all cases. The records include many 

 dead shells. 



Arcella costata Ehrbg. Average number, 48. For the purposes 

 of this paper I have included here all those individuals which possess 

 an angular or ribbed shell. Leidy ('79) refers such forms to A. 

 vulgaris. Individuals of this type are rare, occurring infrequently 

 and in small numbers. It was recorded but 18 times in the 180 

 collections, and the largest number per cubic meter was only 1,187. 

 As in the other species of the genus, the warmer months are favored, 

 fourteen occurrences falling in June-September in water at 70 or 

 above. The other four records are one each in April, October, 

 November, and December. The seasonal range of this form in the 

 plankton thus falls in the main within the period of the maximum 

 abundance of A. vulgaris, of which species it may be but a variant. 



Arcella discoides Ehrbg. Average number, 972. This prevalent 

 species is not in all instances easily separated from A. vulgaris. 

 Indeed, even Leidy ('79) states that it graduates into A. vulgaris, 

 and that he views it as the variety of this species in "which the shell 

 presents a greater proportionate reduction in height compared with 

 the breadth." In the enumeration of our plankton catches, the 

 larger, flatter, and unornamented individuals have been referred to 

 this species. Both the brownish and the hyaline forms should 

 probably, for reasons hereafter given, be included here, and they 

 are so grouped in the present discussion. Thus considered, A. 

 discoides is the most abundant member of the river plankton be- 

 longing to this genus, including two thirds of all the individuals 

 observed. 



This species occurred in almost two thirds of the collections, hav- 

 ing been recorded in 115 of the 180, and more frequently and in 

 larger numbers in the latter half of the five years than it was in the 

 earlier period. This is in part explained by the unusual fluctua- 

 tions of the river levels in 1898, during the maximum summer 

 occurrence of the species. Like the other species of the genus, A. 

 discoides has a period of maximum occurrence in the latter part of 

 summer, as is shown in Table I. Of the 115 occurrences, 55 were in 



